1. Field of the Invention
The present invention relates to viewing devices which are used for surveying, astronomical observation, and so on, such as binoculars and telescopes, etc.
2. Description of the Related Art
Viewing devices, such as binoculars, telescopes and such like, which are used for surveying, astronomical observation, and so on have a viewing optical system which is composed of an eyepiece and an objective optical system. The field curvature of the eyepiece and the objective optical system is respectively corrected. Accordingly, the field curvature of the viewing optical system is totally corrected, so that the field of view obtained through the viewing optical system is fine.
Some viewing devices include a reference plate which is positioned between the objective optical system and the eyepiece. The reference plate is a clear plate, and a reticle, such as cross hairs, or a scale, etc, is formed on the surface by chromium plating. Most reticles are formed radially extending outward on a plane perpendicular to an optical axis of the viewing optical system, intersecting with the optical axis. When a user observes an objective through the eyepiece of the viewing device, the reticle can be viewed with the objective, so that the reticle facilitates the survey and observation.
As described above, generally, the field curvature of the optical system which is provided in the viewing device is corrected for the eyepiece and the objective optical system independently. Accordingly, the field curvature can be corrected for different combinations of eyepieces and objective optical systems.
However, there are some eyepieces which are made for use with predetermined objective optical systems, there are some eyepieces, composed of lenses the number of which is reduced to save cost, and some eyepieces are adapted for a wide-field. The field curvature of these kinds of eyepiece is not always corrected.
Namely, even though the field curvature of an eyepiece is not corrected, the field curvature of the viewing optical system which is composed by the eyepiece and objective lens is corrected. Accordingly, an image of an object, which is viewed through the eyepiece and the objective optical system, is not blurred. However, the reference plate is situated between the objective optical system and the eyepiece, and the reference plate is viewed only through the eyepiece. Accordingly, if the field curvature of the eyepiece is not corrected, the image of the reticle is partially blurred.
Further, the above-mentioned viewing devices can be provided with a frame which determines the field of view obtained when viewed through the eyepiece. The frame masks a peripheral portion of the optical path of luminous flux which passes through the viewing optical system, so that the field of view is determined. On the other hand, as described above, most of the reticles are formed to extend radially from the optical axis. Accordingly, when the field curvature of the eyepiece is not properly corrected, the border of the frame and the reticle can not be clearly viewed at the same time.
If the lens system is designed such that the field curvature of each of the eyepieces and the objective optical systems can be corrected, the above-mentioned problems can be resolved, however, this causes the problem of increased manufacturing cost.